This invention relates to transmission and reception of digital data and more particularly to optimal decoding of digital signals, phase-modulated individually or in quadrature on suppressed carrier signals.
Suppressed carrier transmission is widely used because of the basic fact that no power is wasted in transmitting the carrier since only side bands are transmitted. At the receiver, therefore, the carrier frequency must be restored which has been done by squaring the transmitted signal (after heterodyning) which produces a double frequency term. This squared signal is fed through a bandpass filter or a phase locked loop, then divided down to the proper frequency. This term is then multiplied by the input signal (after heterodyning, before squaring) to produce a signal from which the data can be retrieved.
In the case of quadrature modulation, this complete demoduation sequence is applied to the demodulation separately for each of the modulating signals.